This application claims the priority of German Patent Document 19717825.1, filed Apr. 26, 1997, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for aluminum nitride coating, especially of a cylinder contact surface of a crankcase consisting of an aluminum basic alloy. The invention also relates to an apparatus for performing the method, as well as to a formed product.
In order to reduce static and moved masses and to realize fuel savings associated therewith, as well as to reduce pollutants, light metal alloys based on aluminum are being used increasingly in engine building. An example of reducing moved masses are the aluminum pistons which have been known for a long time. An example of reducing static masses is the use of light metal crankcases made of aluminum basic alloys, for example those of eutectic Al--Si materials, such as Al.sub.8 Si--Cu, made by injection molding.
Cylinder liners are often employed in such engines. At the present time, it is not possible to eliminate the casting of cylinder liners, even though this procedure is not fully satisfactory in all respects. First, the procedure results in a higher engine weight. Second, the connection between the cylinder liner and aluminum alloy represents a sensitive transition for transfer of heat into coolant ducts. If the problem is serious, it is possible for the cylinder liner to come loose, resulting in engine destruction.
In view of the problems mentioned above when cast cylinder liners are used in aluminum crankcases, one may conclude that the use of a sleeveless crankcase would be desirable; this cannot be achieved without additional measures taken with respect to the cylinder contact surface. First, aluminum basic material, in comparison with, for example, iron basic alloys, offers a lower hardness and poor wear resistance to the pistons. As a result, wear is greater, negatively affecting compression, oil stability and usage, and fuel consumption, and general long-term stability of the engine and engine parts such as the crankshaft end bearings. Second, the system composed of an aluminum crankcase and an aluminum piston does not represent a system with optimal tribological properties.
To make aluminum crankcases sleeveless, it is therefore necessary to modify the surface of the contact area by applying a coating with a chemical composition different from that of the aluminum base material which meets strength requirements and has optimal tribological properties. Moreover, the layer must also have satisfactory thermal conductivity values.
Use of a non-oxidic ceramic material, preferably aluminum nitride (AlN), would be advantageous because it would provide these necessary properties. AlN is a hard material that has a thermal conductivity that is comparatively high for ceramic materials. In addition, it has good tribological properties as far as the AlN layer/Al piston pair is concerned. By comparison, the formation of an oxide-ceramic layer, aluminum oxide-Al.sub.2 O.sub.3 for example, has less favorable material properties. It is known that Al.sub.2 O.sub.3 layers obtained by anodization cannot withstand the high alternating mechanical load to the degree and tend to break off.
As for mechanical load-bearing capacity, when AlN is used, comparatively thick layers in the range of at least several orders of 10.times.10.sup.-6 m are required. With respect to coating technology and the anticipated application, this means a high-rate coating method should be employed, preferably a gas phase plasma process.
High application rates in plasma processes are known from the technically established plasma spray process. This is a typical thick-layer method in which a coating material provided in a powder form is melted, at least partially, in the plasma jet of a high-current arc discharge and then applied to a part at a comparatively high kinetic energy. The application of AlN coatings by this conventional plasma spray method is basically possible.
However, the method appears less suitable for the application of the present invention. First, the sprayed-on layer is not optimally anchored to the basic material. Second, a homogeneous material is not obtained. Both factors prove unsatisfactory for long-term resistance to the alternating mechanical loads that occur in the intended application.
In addition, there are problems with performing the process, as specially designed plasma spray heads are necessary for machining the cylinder bores.
Accordingly, the goal of the invention is to provide a method with which a layer, that is firmly anchored to the base material and homogeneous in structure, made of aluminum nitride (AlN) can be applied to an aluminum basic alloy (AlSi.sub.8 Cu, AlSi.sub.9 Cu, AlSi.sub.10 Cu as near-eutectic alloys for example) especially for a crankcase housing in the vicinity of cylinder contact surfaces. With this method, the aluminum nitride layer can also be applied to predetermined contact path structures by means of a honing process. Additional aftertreatment should not be necessary. The method can be used not only in engine manufacturing but also in the repair of used devices.
According to the invention, AlN coating is produced by surface nitriding of the Al basic alloy. The activated nitrogen required for this purpose is produced by a high-pressure plasma process using molecular nitrogen.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.